ECLOGITIC ROCKS IN THE ST-CYR KLIPPE, YUKON, AND THEIR TECTONIC SIGNIFICANCE

Until recently, the Nisutlin allochthonous assemblage, a part of the Yukon - Tanana composite terrane interpreted as trench melange from a late Paleozoic - Mesozoic arc system, was the only tectonic assemblage known to include subducted material in the northern Cordillera. The discovery of eclogitic rocks in two parts of a klippe of the Anvil allochthonous assemblage, which comprises mafic ophiolitic rocks, above the Cassiar terrane west of the Tintina fault confirms other evidence that subducted oceanic crust was also returned to the surface. The eclogitic rocks have been largely retrograded by postsubduction metamorphism. Their existence is interpreted as additional evidence of the link between nappes above the Cassiar terrane and their inferred root, die Teslin suture zone. The Nisutlin and Anvil allochthonous assemblages can now be interpreted, not simply as crustally metamorphosed assemblages with minor, structurally interleaved high-pressure components, but as deeply metamorphosed and intensely strained slices of continental and oceanic crust switched from subducting slab to overriding plate and returned to the surface during collision of the arc with the North American margin.